4. Procedures: i) Screening visit ( - 3 to -1 weeks): Determination of diagnostic eligibility criteria: medical history, complete physical examination (weight, heart rate, blood pressure, respiratory rate); familiarization with assessment procedures, rating scales for: tics, the Yale Global Tic Severity Scale (YGTSS), Clinical Global Impression-Severity Scale (CGI-S), obsessive-compulsive problems, Children's Yale-Brown Obsessive Compulsive Scale (CY-BOCS); attention deficit hyperactivity disorder, DuPaul ADHD Rating Scale, depression, Child Depression Inventory- Short Version (CDI-S); and anxiety, the Multidimensional Anxiety Scale for Children (MASC). Informed consent and child assent will be obtained. Urine (urine analysis) and blood (comprehensive metabolic panel, complete blood count, and plasma amino acids) will be obtained. Females who have begun to menstruate will be screened with a blood pregnancy test. Parents will be told the results of these tests. If the patient is uncomfortable with our telling the results of the pregnancy test to her parent(s), then they may decide not to take part in this study.